1. Field of the Invention
The present invention relates to semiconductor processing, and more particularly, the present invention relates to the etch process by which semiconductor material is etched out leaving well-defined features. Still more particularly, the present invention relates to a system, method and software program product for accurately determining the changes in a signal that are indicative of an endpoint of the etching process.
2. Description of Related Art
There are many steps involved in the processing of a wafer, of which etching is one of the crucial steps. Etching is a process whereby a selected area on a wafer surface is removed so as to make a desired pattern on the surface. Plasma etching can accurately remove patterns of very small dimension on the surface of a semiconductor wafer. Reactive Ion Etching (RIE) is an etching technique in which radio frequency radiation in a low pressure gas ionizes the gas and dissociates the gas molecules into more reactive species.
FIG. 1 is a cross-sectional illustration of an etcher in which the RIE process may be performed. FIG. 1 is a diagram of an exemplary etcher intended only to aid in describing etching principles useful in understanding the description of the present invention and not intended to faithfully represent any actual etcher. Etcher 100 is a plasma etch reactor in which the RIE process is confined within chamber 102. In operation, plasma 118 is produced in chamber 102 when etching gas 122 enters reaction chamber 102 and is ionized by the application of an electric field established between cathode 110 and anode 112. As etching gas 122 is ionized into plasma 118, the respective velocity of electrons and ions are significantly different due to the difference in their masses. A typical etcher 100 uses anode 112 at ground potential and cathode 110 connected to the RF generator 114 and biased above ground. Wafer 130 is placed on the platen. Gas molecules of etching gas 122 are accelerated to the substrate surface of wafer 130 toward cathode 110 due to the difference in potential across the electrodes. Wafer 130 is bombarded with reactive positive ions created in the plasma which causes atoms of the substrate to be sputtered and usually react chemically with etching gas 122, thereby removing the top layer of material on wafer 130. The newly-formed gases 126 are removed from chamber 102 by vacuum system 125 through exhaust port 124.
The etching step of wafer production is an integral part of semiconductor manufacturing; however, equally important to accurate etching is detecting the precise point in time the etching process has ended, i.e. the xe2x80x9cendpoint.xe2x80x9d The endpoint of the etching process is where all etched feature patterns are fully delineated and undercutting of the substrate is held to a minimum. Typically, endpoint detection mechanisms determine the endpoint of an etch process by distance determinations or by optical emission. A laser interferometer (not shown in the figure) reflects laser radiation off wafer 130 during processing and the thickness of the etched layer is determined by the interference of the reflected light. Alternatively, the optical emissions of the reaction products of the etching process are used to determine an endpoint.
Emission collection and processing mechanism 150 receives and pre-processes light emitted by the plasma into a form usable by an endpoint detection mechanism (not shown). Initially, collimating and focusing optics 152 receive light 154 from inside chamber 102 and transmit it, usually through an optical fiber, to optical emission analyzer 156. One type of optical emission analyzer 156, a monochromator, monitors the intensity of a single wavelength of light 154 from the exhaust gases and outputs signal 158 based on the intensity of the light at the wavelength being monitored. Generally, it is expected that the intensity of the light at this wavelength will change at the endpoint of the etching process, and thus the output signal 158 indicates that transition.
Accurately detecting the endpoint of an etch process by analyzing the optical emission of the reaction products depends on identifying an optimal discrete wavelength, usually associated with a reactant species, that exhibits a quantifiable intensity change of the light associated with the endpoint transition. Although the intensity of light from many reactant species decreases at the etching process endpoint, the light from some other species increases in intensity. Many factors have a detrimental effect on signal detection that must be compensated for, e.g. low amplitude of transmitted light energy 154 due to a dirty view port, spurious noise from plasma fluctuations that masks the endpoint signal, unsteady electrical fields from the electrodes, electronics malfunctions, or inaccurate optical measurements.
As fine line patterning becomes more prevalent and the percentage of area to be etched becomes smaller, accurate endpoint detection becomes more difficult. As feature sizes decrease, the percent of the wafer open area also decreases, requiring plasma etch endpoint detection systems to be more sensitive and accurate. Traditional endpoint detection systems that monitor one or two wavelengths do not generate enough information for successful endpoint determination when open areas drop below a nominal surface percentage.
In another alternative, collimating and focusing optics 152 receive light from reactant species inside chamber 102 and feeds the entire spectrum of light over a substantially broad range of wavelengths 154 to optical emission analyzer 156. Optical emission analyzer 156 is a spectrometer which is capable of monitoring multiple discrete wavelengths. Using processing functionality in optical emission analyzer 156, the operator can then select the most appropriate sets of wavelengths for reactant species associated with a particular etching process and generate output 158 from the intensities of the selected wavelengths. Monitoring intensities of multiple wavelengths gives an operator increased flexibility. The disadvantage is that the complexity increases for endpoint detection.
FIG. 1 further depicts etcher 100 as having magnets 142 on rotating magnet housing 140 for magnetically enhanced RIE etching. Magnets 142 revolve around chamber 102 causing plasma 118 to follow the magnetic field associated with magnets 142. This produces a more homogenous etch of the surface. However, magnets 142 induce one or more bright spots in plasma 118 that revolve with magnet housing 140. Optical emission analysis of the etch process becomes more challenging because the intensity of the light 154 now is periodic with the rotation of a mechanical device and is not based solely on the reactant processes. Here it is necessary for the operator to have expert knowledge of plasma chemistry and/or spectroscopy. The operator must also understand how the rotating magnets degrade the endpoint signal. Understanding how to do this optical emission analysis to find the endpoint using multiple sets of wavelength from the output of a spectrometer has been heretofore unknown.
The present invention is directed to a system, method and software product for creating a predictive model of the endpoint of etch processes using Partial Least Squares Discriminant Analysis (PLS-DA). Initially, intensity readings for discrete wavelengths in a spectrum are collected from a calibration wafer using optical emission spectroscopy (OES). Intensity values in the OES data may represent a signal that is non-periodic or periodic with time. Periodic signals may be sampled synchronously or non-synchronously with the period of a signal. Initially, the OES data is arranged in a spectra matrix X having one row for each data sample.
The OES data is processed to remove transients that occur during the startup and shutdown of the etch process. Wavelength regions are selected with desirable endpoint transition qualities, such as sharpness, and wavelengths with saturated intensity values are wholly removed from the processed OES data.
A preview endpoint signal is plotted using the selected wavelength regions and/or PCA analysis on the spectra matrix X. Regions of stable intensity values on the endpoint plot, that are associated with either the etch region or the post-etch region, are identified by sample number. An X-block is created from the processed OES data samples associated with the two regions of stable intensity values. Non-periodic OES data and asynchronously sampled periodic OES data are arranged in an X-block by one sample per row. Synchronously sampled periodic OES data are arranged in the X-block by one period per row. A y-block is created for classifying the features of the etch process by using binary partitioning; here, these features are the regions of stable intensity values associated with the etch. The y-block is created by assigning a discriminate variable value of xe2x80x9c1xe2x80x9d to OES samples in the class of the etch region and assigning a discriminate value of xe2x80x9c0xe2x80x9d to samples not in that class. A b-vector is obtained by regression from the X- and y-blocks using PLS and is validated using the processed OES data in spectra matrix X. Various other vectors are obtained from the validated b-vector and are used with the appropriate algorithm to process real-time OES data from a production etch process to detect endpoint.